it's possible for characters to act like another class due to fascination with it or being forced to do so (ex. rose with the witch class and jane with the heir class), right? would it be possible for a player to act like several other classes, maybe because of an identity crisis or hating their "real" class or something, to the point that it causes a problem to their sense of self and capability to act? what class would be most likely to face this kind of crisis?

Yeah this is the basis of the Roleplay System. Chances are they wouldn’t be acting as more than one Class at ONCE but flipping from one to another could be feasible if they really are opposed to their Class or being forced to act in different ways. I would say a Knight may be like this- Trying to serve their friends in every situation, it ends up making them act differently for every given situation. Maybe a Knight of Heart or Mind.